1. Field of the Invention
The present invention relates to a signal-quality evaluation device for evaluating the quality of optical signals, a signal adjustment method, and an optical-signal evaluation system and an optical transmission system which are equipped with such signal-quality evaluation device.
2. Description of the Related Art
In a future optical transmission system, an optimal condition for transmitting optical signals will tend to change according to the system structure that becomes more complicated due to an increase in transmission capacity. Therefore, adjustment means (e.g. dispersion compensation means) for constantly optimizing the transmission condition is necessary. Furthermore, in order to utilize such adjustment means effectively, a device that evaluates and monitors the quality of an optical signal is also necessary.
In order to evaluate the quality of an optical signal, a certain parameter must be extracted from the optical signal being transmitted. A typical example of this parameter is a bit error rate (BER). The bit error ratio is a ratio of error in which an optical signal is recognized as level 1 when the actual level is level 0 or recognized as level 0 when the actual level is level 1, such error being caused by noise or waveform distortion of the optical signal due to the transmission thereof. Since a BER is directly related to the quality of an optical signal, it is reliable as a parameter to be used for an evaluation. Actually, there is an example in which a BER is used as a reference for implementing dynamical adjustment in variable dispersion compensators. However, determining a BER requires a long period of time for measurement, and especially if the BER is small, the time required for measurement is even longer.
A parameter that is known as similar to a BER is a Q-factor (I. Shake, et al., IEEE Photonics Technology Letters, Vol. 13, No. 4, pp. 385 to 387 (2001)). The Q-factor also requires time for determining the amplitude distributions of optical signals, that is, the time required for evaluation is long.
Furthermore, the quality of an optical signal can also be evaluated by detecting cumulative dispersion (Y. Takushima, et al., IEEE Photonics Technology Letters, Vol. 15, No. 6, pp. 870 to 872 (2003), and K. J. Park, et al., IEEE Photonics Technology Letters, Vol. 15, No. 6, pp. 873 to 875 (2003)). According to this method, when an optical signal is to be transmitted, the optical signal is subject to frequency modulation or slight intensity modulation. The modulated component is then extracted at a receiving side. Based on the detection result of the modulated component, the cumulative dispersion of the optical signal is determined. Thus, the quality of the optical signal is evaluated on the basis of the cumulative dispersion. In this method, however, since the optical signal must be processed at a transmitting side, it is difficult to apply this method to an optical network that has a plurality of transmitting sides and a plurality of receiving sides. Moreover, subjecting an optical signal to such a processing could also cause waveform distortion.
A method of monitoring the quality of an optical signal by evaluating the distortion itself of an optical signal is also known (P. S. Westbrook, et. al., IEEE Photonics Technology Letters, Vol. 14, No. 3, pp. 346 to 348 (2002), and Z. Pan, et al., OFC2001 WH5). According to this method, a clock frequency component of an optical signal is extracted at a receiving side, and the quality of the optical signal is evaluated based on this extraction result. Alternatively, according to the principle that the degree of spectral broadening depends upon an optical-signal pulse width, the quality of an optical signal is evaluated by measuring the degree of spectral broadening caused by self-phase modulation to which an optical signal is subjected. Although these methods are advantageous in that the optical signal need not be processed at a transmitting side, these methods are inferior in the flexibility of application in terms of signal format of optical signals: specifically, the method of Westbrook, et al. is not applicable to CSRZ signal format, and the method of Pan, et al. is not applicable to NRZ signal format.